1. Field of the Invention
The invention is related to the technical field of wireless communications systems. More particularly, the invention is related to the technical field of coordinated multi-point transmission and reception in wireless communications systems.
2. Description of the Related Art
Due to mobile communications technology advancements in recent years, various communications services, such as voice call services, data transfer services, and video call services, etc., may be provided to users regardless of their locations. Most mobile communications systems are multiple access systems in which access and wireless network resources are allocated to multiple users. The multiple access technologies employed by the mobile communications systems include the 1x Code Division Multiple Access 2000 (1x CDMA 2000) technology, the 1x Evolution-Data Optimized (1x EVDO) technology, the Orthogonal Frequency Division Multiplexing (OFDM) technology, the Long Term Evolution (LTE) technology, or others. Evolved from the LTE, the LTE-Advanced is a major enhancement of the LTE standard. The LTE-Advanced should be compatible with LTE equipment, and should share frequency bands with the LTE communications system.
One of the important LTE-Advanced benefits is its ability to take advantage of advanced topology networks. FIG. 1 shows an exemplary deployment of a heterogeneous network according to the prior art. The optimized heterogeneous networks have a mix of macrocells (e.g. the macrocell 10) and low power nodes, such as remote radio heads (RRHs) (e.g. the RRH 20), picocells (e.g. the picocell 30), femtocells (e.g. the femtocell 40) and relay nodes (RNs) (e.g. the relay node 50). Remote radio head is applied for the purpose of inter-cell radio resource management and coverage enhancement, which is a sufficiently small device so that it can easily be installed near antennas. Considering the transmit power of RRH, not only can a low power RRH be employed, but also a high power RRH, having high powers as that of an evolved NodeB (eNB), can be employed in the radio access network (RAN). Therefore, for LTE-Advanced, the RRH may be applicable not only to picocells but also to macrocells. For a RRH, the backhaul link is established on X2 interface, e.g. by optical fiber. Relay node is applied for improving the cell-edge throughput, group mobility and temporary network deployment. For a relay node, the backhaul link is established through air interface. Coordinated multi-point (CoMP) transmission and reception is a novel technique developed under the LTE-Advanced, as a tool to improve the coverage at the cell-edge of an evolved node B (eNB) and/or to increase system throughput in both high load and low load scenarios. CoMP transmission and reception implies dynamic coordination among multiple geographically separated transmission points. The core technology of CoMP may be categorized into Joint Processing (JP) and Coordinated Scheduling/Beamforming (CS/CB).
For Joint Processing (JP), data is available at each point in a CoMP cooperating set. The CoMP cooperating set is a set of (geographically separated) points directly or indirectly participating in data transmission to the user equipment (UE). Note that this set may or need not be transparent to the UE. The CoMP transmission point(s) is the point or set of points actively transmitting data to the UE. For Coordinated Scheduling/Beamforming (CS/CB), data is only available at the serving cell (data transmission from that point) but user scheduling/beamforming decisions are made with coordination among cells corresponding to the CoMP cooperating set, where the serving cell is the cell transmitting physical downlink control channel (PDCCH) assignments.
The Joint Processing (JP) may further be categorized into Joint Transmission (JT) and Dynamic Cell Selection (DCS). For Joint Transmission (JT), data is transmitted from multiple points (part of or entire CoMP cooperating set) at a time. In other words, data to a single user equipment (UE) is simultaneously transmitted from multiple transmission points, so as to improve the received signal quality and/or actively cancel interference for other UEs. For Dynamic Cell Selection (DCS), data is transmitted from one point at a time (within CoMP cooperating set). In other words, data is spread in multiple transmission points of the entire CoMP cooperating set, but there is only one transmission point transmitting data to the UE at a time.
A cluster is defined as a group of CoMP transmission/reception points. How to set the CoMP cooperating sets, i.e. clustering, is expected to play an important role in the CoMP operation and performance. FIG. 2 shows an exemplary deployment of a cluster in a CoMP communications system according to the prior art. In FIG. 2, a center site 110 is surrounded by and connected to six remote devices 120-1 to 120-6, to form a cluster 100. The center site 110 may be an eNB, the remote devices 120-1 to 120-6 may be RRHs, and the connection lines may be the optical fibers. The center site 110 is operative to serve one or more UEs, e.g. 250 and 260, located under the coverage of the cluster 100, and the remote devices 120-1 to 120-6 are controlled by the center site 110 to simultaneously (that is, when the CoMP transmission or reception is performed) or non-simultaneously transmit downlink signals to the one or more UEs and/or receive uplink signals from the one or more UE.
FIG. 3 shows another exemplary deployment of a cluster in a CoMP communications system according to the prior art. In FIG. 3, a center site is surrounded by and connected to eighteen remote devices to form a cluster 200. Regardless of which kind of cluster deployment is, the UEs located within coverage of a cluster can benefit from the intra-site CoMP operation of the cluster. However, a UE located near boundaries of a cluster (such as the coverage boundary) will not be able to gain the benefit of intra-site CoMP operations. For example, referring back to FIG. 2, the UE 250 can benefit from intra-site the CoMP operation of the cluster because both of the remote devices 120-4 and 120-5 can simultaneously transmit or receive signals to or from the UE 250 under the control of the center site 110. However, for the UE 260 located near the coverage boundary of the cluster 100, the center site 110 generally assigns the remote device 120-3 to transmit or receive signals to or from the UE 260 since only the remote device 120-3 is geographically close to the UE 260. In this manner, even if the UE 260 is under the coverage of the cluster deployed to support the CoMP operations, UE 260 is still unable to gain the benefit of intra-site CoMP operations.
To solve this problem, a novel CoMP communications system and methods for controlling CoMP transmission and reception in a wireless communications system is desirable.